1. Field of the Invention
The present invention relates generally to a composite membrane and a method of making the composite membrane. In particular, the present invention relates to a porous membrane that is treated to provide oleophobic properties to the membrane and to a method of treating the membrane.
2. Description of Related Art
It is known that technical fabrics must be suitable for use in demanding applications. Examples of such demanding applications include filter elements, outerwear garments and apparel, footwear, tents, sleeping bags, protective garments, clean room garments, surgical drapes, surgical gowns, other types of barrier wear and allergen barrier products. The technical fabrics often include a film or membrane to protect the fabric user from an external condition or environment and/or protect the external environment from contamination by the user. The film or membrane may be made from any suitable material or structure and in any suitable manner.
A known material for the membrane that has proven particularly suitable for such demanding applications is made of an expanded polytetrafluoroethylene (“ePTFE”) material. The ePTFE membrane is typically microporous and laminated to at least one other material, such as a textile base or shell fabric. The resulting membrane and fabric laminate can be used to manufacture any number of finished products to meet the demands of the particular application.
It is known that an ePTFE membrane is air permeable and moisture vapor transmissive, yet resistant to wind and liquid penetration at moderate pressures. However, the ePTFE membrane tends to absorb oils and certain contaminating agents, such as body oils contained in perspiration, fatty substances or detergent-like contaminants. When the ePTFE membrane becomes contaminated by absorbing the oils or other contaminating agents, the membrane may no longer effectively resist liquid penetration.
One known approach to rendering an ePTFE membrane resistant to contamination by absorbing oils or contaminating agents includes applying a layer of polyurethane onto, or partially into, the ePTFE membrane, as disclosed in U.S. Pat. No. 4,194,041. A membrane with a polyurethane layer is generally contaminating agent resistant and has relatively high moisture vapor transmission rates. However, air may not freely permeate through the polyurethane layer. It is known that some degree of air permeability is desirable to increase user comfort. It is also known that the polyurethane layer must be wet in order to effectively transmit moisture vapor which can feel cold, wet and “clammy” to the user.
Another known approach to contamination resistance is to coat surfaces defining the pores in a porous membrane with a fluoroacrylate monomer and a polymerization initiator, as disclosed in U.S. Pat. No. 5,156,780. The initiator is activated to polymerize the monomer in situ to coat surfaces defining the pores in the membrane. This approach provides a membrane that is somewhat air permeable and resistant to absorbing oils and contaminating agents. However, this approach requires relatively expensive equipment and materials, such as an ultraviolet curing station and a nearly oxygen-free or inert atmosphere, to process and polymerize the monomer once it is applied to the membrane. Furthermore, this approach requires solvents, cross-linking reactants and/or initiators that may be environmentally unsound and difficult to obtain.
Yet another known approach is to coat a microporous membrane with an organic polymer having recurring pendant fluorinated organic side chains, as disclosed in U.S. Pat. No. 5,539,072. An aqueous dispersion carries the polymer and cannot enter the pores of the membrane. A relatively expensive fluorosurfactant is used in amounts that may be difficult to completely remove from the membrane in order to permit the polymer in the aqueous dispersion to enter the pores of the membrane.
Thus, a need exists to provide a membrane that is air permeable, moisture vapor transmissive, wind and liquid penetration resistant, durably resists absorbing oils and certain contaminating agents, is relatively inexpensive and easy to manufacture, made from readily available materials and does not require relatively expensive equipment, processes or materials. There is also a need to limit the agglomeration and “settling” of dispersed particles or “solids” in a coating that is to be applied to the membrane for a predetermined duration.